Aren't multi-az deployments more expensive? That would be a valid reason not to check this checkbox, if your business can survive a bit of downtime here and there.
Most of that expense is just the cost of a hot failover, but there is some additional cost around inter-AZ data transfer. If someone is not checking the boxes for cost reasons, I would be surprised if they had failovers in the same AZ. It seems more likely they just don't have failovers.